Modern traffic control signals, or traffic lights, are commonly controlled by a computer based controller. Because the controller typically receives its power from a utility line, the controller is subjected to power abnormalities such as voltage spikes, voltage fluctuations as well as outright power failures, which can cause the controller to fail and lead to a complete shut down of the traffic lights. Thus a system is needed which filters voltage spikes and voltage fluctuations and provides backup power in the event of a power failure.
While controller failures have been proposed which initiate an alarm upon detection of a problem, this is often too late and can also be very dangerous. For example, if two or more cars are approaching an intersection from different directions, and the traffic lights suddenly go out, the approaching cars would not know who had the right-of-way and could crash. Thus a system is needed which provides for an orderly shut down of the traffic lights upon loss of power.
Various systems have been proposed for providing computers with back-up power, such as can be provided by a bank of batteries coupled in series and an invertor. Such systems often include battery chargers for charging the bank of batteries to a predetermined float voltage, the float voltage being determined by the sum of the voltages of the batteries. However, if one, or more, of the batteries, or cells thereof, are defective and, hence, effectively a short, the total voltage across the bank of batteries will never reach the float voltage to shutoff the battery charger, resulting in a damaging overcharging of the remaining good batteries.
Further, when the utility power returns, it is necessary for the controller to transparently switch from the back up power to the utility power, i.e., the output of the invertor must be in-phase with the utility power.
Still further, systems often shut down when the line voltage is determined to be too low. However, utility lines often have relatively high impedance, and line conditioners to condition utility power often are highly inductive. The high inductance results in a large inrush of current upon a restart of the controller, and this large inrush current traveling through the high impedance utility line results in a short term voltage drop which can trick the system into shutting down.
Finally, inverters typically include power transistors, such as Darlington transistors. To monitor the condition of a load coupled to the invertor, devices have been proposed which monitor the current through the power transistors when conducting. This, however does not always provide an accurate indication of the condition of the load.
Applicants' invention is provided to solve these and other problems.